Code division multiple access (CDMA) communication systems provide wireless communications between a base station and one or more mobile or portable subscriber units. The base station is typically a computer-controlled set of transceivers that are interconnected to a land-based public switched telephone network (PSTN). The base station further includes an antenna apparatus for sending forward link radio frequency signals to the mobile subscriber units and for receiving reverse link radio frequency signals transmitted from each mobile unit. Each mobile subscriber unit also contains an antenna apparatus for the reception of the forward link signals and for the transmission of the reverse link signals. A typical mobile subscriber unit is a digital cellular telephone handset or a personal computer coupled to a cellular modem. In such systems, multiple mobile subscriber units may transmit and receive signals on the same center frequency, but unique modulation codes distinguish the signals sent to or received from individual subscriber units.
In addition to CDMA, other wireless access techniques employed for communications between a base station and one or more portable or mobile units include those described by the Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard and the industry developed wireless Bluetooth standard. All such wireless communications techniques require the use of an antenna at both the receiving and transmitting site. It is well-known by experts in the field that increasing the antenna gain in any wireless communication system has beneficial affects.
A common antenna for transmitting and receiving signals at a mobile subscriber unit is a monopole antenna (or any other antenna with an omnidirectional radiation pattern). A monopole antenna consists of a single wire or antenna element that is coupled to a transceiver within the subscriber unit. Analog or digital information for transmission from the subscriber unit is input to the transceiver where it is modulated onto a carrier signal at a frequency using a modulation code (i.e., in a CDMA system) assigned to that subscriber unit. The modulated carrier signal is transmitted from the subscriber unit antenna to the base station. Forward link signals received by the subscriber unit antenna are demodulated by the transceiver and supplied to processing circuitry within the subscriber unit.
The signal transmitted from a monopole antenna is omnidirectional in nature. That is, the signal is sent with approximately the same signal strength in all directions in a generally horizontal plane. Reception of a signal with a monopole antenna element is likewise omnidirectional. A monopole antenna does not differentiate in its ability to detect a signal in one azimuth direction versus detection of the same or a different signal coming from another azimuth direction. Also, a monopole antenna does not produce significant radiation in the elevation direction. The antenna pattern is commonly referred to as a donut shape with the antenna element located at the center of the donut hole.
A second type of antenna employed by mobile subscriber units is described in U.S. Pat. No. 5,617,102. The directional antenna comprises two elements which are mounted on the outer case of a laptop computer, for example. A phase shifter attached to each element imparts a phase angle delay to the input signal, thereby modifying the antenna pattern (which applies to both the receive and transmit modes) to provide a concentrated signal or beam in the selected direction. Concentrating the beam increases the antenna gain and directivity. The dual element antenna of the cited patent thereby directs the transmitted signal into predetermined sectors or directions to accommodate for changes in orientation of the subscriber unit relative to the base station, thereby minimizing signal loss due to the orientation change. In accordance with the antenna reciprocity theorem, the antenna receive characteristics are similarly effected by the use of the phase shifters.
CDMA cellular systems are interference limited systems. That is, as more mobile or portable subscriber units become active in a cell and in adjacent cells, frequency interference increases and thus bit error rates also increase. To maintain signal and system integrity in the face of increasing error rates, the system operator decreases the maximum data rate available to one or more users, or decreases the number of active subscriber units, which thereby clears the airwaves of potential interference. For instance, to increase the maximum available data rate by a factor of two, the number of active mobile subscriber units is halved. However, this technique cannot generally be employed to increase data rates due to the lack of service priority assignments to the subscribers. Finally, it is also possible to avert excessive interference by using directive antennas at both (or either) the base station and the portable units. Typically, a directive antenna beam pattern is achieved through the use of a phased array antenna. The phased array is electronically scanned or steered to the desired direction by controlling the phase angle of the signal input to each antenna element. However, phased array antennas suffer decreased efficiency and gain as the element spacing becomes electrically small compared to the wavelength of the received or transmitted signal. When such an antenna is used in conjunction with a portable or mobile subscriber unit, generally the antenna array spacing is relatively small and thus antenna performance is correspondingly compromised.
In a communication system in which portable or mobile units communicate with a base station, such as a CDMA communication system, the portable or mobile unit is typically a hand-held device or a relatively small device, such as, for instance, the size of a laptop computer. In some embodiments, the antenna is inside or protrudes from the device housing or enclosure. For example, cellular telephone handsets utilize either an internal patch antenna or a protruding monopole or dipole antenna. A larger portable device, such as a laptop computer, may have the antenna or antenna array mounted in a separate enclosure or integrated into the laptop case. A separate antenna may be cumbersome for the user to manage as the communications device is carried from one location to another. While integrated antennas overcome this disadvantage, they are generally in the form of protrusions from the communications device, except for a patch antenna. These protrusions can be broken or damaged as the device is moved from one location to another. Even minor damage to a protruding antenna can drastically change it's operating characteristics.